Optical system for endoscope

ABSTRACT

An optical system for an endoscope includes a negative lens-group, an aperture diaphragm and a positive lens-group in this order from an object side, wherein the negative lens-group includes a negative first-lens and a negative second-lens in this order from the object side; and wherein the optical system meets conditional expressions (1) and (2):
 
0.05&lt; H (100)×( nd 01−1)/ Rf 01&lt;0.38  (1)
 
and
 
0.5&lt; enp /FL&lt;2.5  (2),
 
where: H(100) is a height at which a principal ray with a field angle of incidence of 100 degrees falls on a surface on the object side of the first lens; Rf01 is a curvature radius of the surface on the object side of the first-lens; nd01 is a refractive index of the first-lens; FL is a focal length of an entire system; and enp is a distance between an entrance pupil and the surface on the object side of the first lens.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation application of PCT/JP2012/072926filed on Sep. 7, 2012 and claims benefit of Japanese Application No.2011-221667 filed in Japan on Oct. 6, 2011, the entire contents of whichare incorporated herein by this reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical system for an endoscope thathas a field angle of no less than 200 degrees.

2. Description of the Related Art

Since endoscopes are intended to, for example, observe an inside of abody cavity, optical systems for endoscopes are demanded to have a smallsize and a wide angle, and in particular, optical systems for urologicalendoscopes need to be small in a radial direction.

As examples of optical systems having a field angle of no less than 200degrees, which can be employed in endoscopes, those described inJapanese Patent Application Laid-Open Publication No. 2007-279632(Patent Literature 1), Japanese Patent Application Laid-Open PublicationNo. 2008-058387 (Patent Literature 2), Japanese Patent ApplicationLaid-Open Publication No. 2005-227426 (Patent Literature 3) and JapanesePatent Application Laid-Open Publication No. 2009-276371 (PatentLiterature 4) are known.

Among them, the optical systems disclosed in Patent Literatures 1 to 3each include a negative first lens, a negative second lens, a positivethird lens, an aperture diaphragm and a positive lens group in thisorder from the object side and have a field angle of 200 degrees.

Also, the optical system disclosed in Patent Literature 4 includes anegative first lens, a negative second lens, a filter, an aperturediaphragm and a positive lens group in this order from the object sideand has a field angle of 200 degrees.

SUMMARY OF THE INVENTION

An optical system for an endoscope according to an aspect of the presentinvention includes a negative lens group, an aperture diaphragm and apositive lens group in this order from an object side, wherein thenegative lens group includes a negative first lens and a negative secondlens in this order from the object side; and wherein the optical systemmeets conditional expressions (1) and (2):0.05<H(100)×(nd01−1)/Rf01<0.38  (1)and0.5<enp/FL<2.5  (2),where: H(100) is a height at which a principal ray with a field angle ofincidence of 100 degrees falls on a surface on the object side of thefirst lens; Rf01 is a curvature radius of the surface on the object sideof the first lens; nd01 is a refractive index of the first lens; FL is afocal length of an entire system; and enp is a distance between anentrance pupil and the surface on the object side of the first lens.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram illustrating a configuration of an optical systemfor an endoscope according to a first example of an embodiment of thepresent invention, along an optical path;

FIG. 1B includes diagrams illustrating a spherical aberration, anastigmatic aberration, a distortion aberration and a chromaticaberration of magnification in the optical system for an endoscopeaccording to the first example of the embodiment;

FIG. 2A is a diagram illustrating a configuration of an optical systemfor an endoscope according to a second example of the embodiment, alongan optical path;

FIG. 2B includes diagrams illustrating a spherical aberration, anastigmatic aberration, a distortion aberration and a chromaticaberration of magnification in the optical system for an endoscopeaccording to the second example of the embodiment;

FIG. 3A is a diagram illustrating a configuration of an optical systemfor an endoscope according to a third example of the embodiment, alongan optical path;

FIG. 3B includes diagrams illustrating a spherical aberration, anastigmatic aberration, a distortion aberration and a chromaticaberration of magnification in the optical system for an endoscopeaccording to the third example of the embodiment;

FIG. 4A is a diagram illustrating a configuration of an optical systemfor an endoscope according to a fourth example of the embodiment, alongan optical path;

FIG. 4B includes diagrams illustrating a spherical aberration, anastigmatic aberration, a distortion aberration and a chromaticaberration of magnification in the optical system for an endoscopeaccording to the fourth example of the embodiment;

FIG. 5A is a diagram illustrating a configuration of an optical systemfor an endoscope according to a fifth example of the embodiment, alongan optical path;

FIG. 5B includes diagrams illustrating a spherical aberration, anastigmatic aberration, a distortion aberration and a chromaticaberration of magnification in the optical system for an endoscopeaccording to the fifth example of the embodiment;

FIG. 6A is a diagram illustrating a configuration of an optical systemfor an endoscope according to a sixth example of the embodiment, alongan optical path; and

FIG. 6B includes diagrams illustrating a spherical aberration, anastigmatic aberration, a distortion aberration and a chromaticaberration of magnification in the optical system for an endoscopeaccording to the sixth example of the embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will be described below withreference to the drawings.

Embodiment

An optical system for an endoscope according to the present embodimentincludes a negative lens group, an aperture diaphragm and a positivelens group in this order from the object side. Furthermore, the negativelens group includes a negative first lens and a negative second lens inthis order from the object side. Consequently, a position of an entrancepupil moves to the object side, enabling reduction in diameter of thenegative first lens. In addition, the number of lenses in the respectivegroups included in the optical system for an endoscope can be reduced,enabling reduction in total length and costs.

Here, the optical system for an endoscope meets conditional expressions(1) and (2):0.05<H(100)×(nd01−1)/Rf01<0.38  (1);and0.5<enp/FL<2.5  (2),where H(100) is a height at which a principal ray with a field angle ofincidence of 100 degrees falls on a surface on the object side of thefirst lens, Rf01 is a curvature radius of the surface on the object sideof the first lens, nd01 is a refractive index of the first lens, FL is afocal length of an entire system, and enp is a distance between theentrance pupil and the surface on the object side of the first lens.

First, conditional expression (1) is a conditional expression relatingto the height at which a principal ray with a field angle of incidenceof 100 degrees falls on the surface on the object side of the firstlens, and the refractive index and a curvature of the first lens.Conditional expression (1) is a part of Abbe's invariant and is amathematical expression that defines an amount of change in angle of alight ray before and after refraction at the first surface, that is, anamount of refraction. If H(100)×(nd01−1)/Rf01 exceeds the upper limit inconditional expression (1), the amount of refraction is excessivelylarge, unfavorably resulting in easy occurrence of an off-axisaberration. Also, if H(100)×(nd01−1)/Rf01 falls below the lower limit inconditional expression (1), the amount of refraction is excessivelysmall, unfavorably resulting in an increase in diameter of the lens.

Next, conditional expression (2) is a conditional expression relating tothe entrance pupil. As long as enp/FL falls within the range inconditional expression (2), a proper entrance pupil position isachieved, enabling a decrease in size without occurrence of variousaberrations in a large amount. On the other hand, if enp/FL exceeds theupper limit in conditional expression (2), the lens is unfavorablyenlarged in the radial direction. Also, if enp/FL falls below the lowerlimit in conditional expression (2), it is necessary to significantlyincrease powers of the negative first lens and the negative second lens,unfavorably resulting in easy occurrence of various aberrations in alarge amount.

Note that conditional expressions (1) and (2), which have been describedabove, are preferably:0.1<H(100)×(nd01−1)/Rf01<0.37  (1)′;and1.0<enp/FL<2.0  (2)′,and more preferably:0.15<H(100)×(nd01−1)/Rf01<0.36  (1)″;and1.4<enp/FL<1.6  (2)″.

The optical system for an endoscope may further meet conditionalexpressions (3) and (4):0.5<H(100)/FL<3.5  (3);and1.1<(r1+r2)/(r1−r2)<1.9  (4),where r1 is the curvature radius of the surface on the object side ofthe first lens and r2 is a curvature radius of a surface on the imageside of the first lens.

Conditional expression (3) is a conditional expression relating to theheight at which a principal ray with a field angle of incidence of 100degrees falls on the surface on the object side of the first lens. IfH(100)/FL falls below the lower limit in conditional expression (3), thelens is extremely excessively small in the radial direction, causing theneed to increase the power of the negative first lens, unfavorablyresulting in occurrence of axial and off-axis aberrations. Also, ifH(100)/FL exceeds the upper limit in conditional expression (3), thelens is unfavorably enlarged in the radial direction.

Conditional expression (4) is a conditional expression relating to ashape factor of the first lens in the negative lens group. As long as(r1+r2)/(r1−r2) falls within the range in conditional expression (4), anecessary negative power can be provided while a wide field angle isprovided. If (r1+r2)/(r1−r2) falls below the lower limit in conditionalexpression (4), the refractive power of the negative first lens isunfavorably lowered. Also, if (r1+r2)/(r1−r2) exceeds the upper limit inconditional expression (4), the productivity of the lens is unfavorablylowered substantially.

Note that conditional expressions (3) and (4), which have been describedabove, are preferably:1.0<H(100)/FL<2.0  (3)′;and1.2<(r1+r2)/(r1−r2)<1.8  (4)′,and more preferably:1.8<H(100)/FL<2.1  (3)″;and1.3<(r1+r2)/(r1−r2)<1.7  (4)″.Where a lens system incident angle ω=100, the optical system for anendoscope may further meet conditional expression (5):0.00<|EX(ω)|/ω<0.03  (5),where EX(ω) is an angle at which a principal ray incident at theincident angle ω exits from the optical system.

Conditional expression (5) is an expression relating to an exit anglerelative to an incident angle in the optical system, and represents atelecentricity (image-side telecentricity) of the optical system for anincident angle of 100 degrees. If the telecentricity is lost, effectssuch as a decrease in brightness in the periphery and/or color mixtureoccur due to the oblique incidence property of image pickup devices suchas CCD and C-MOS. Conditional expression (5) is a conditional expressionfor preventing such deterioration in image quality.

The optical system for an endoscope may further meet conditionalexpression (6):−8.0<F_front/F_rear<−0.1  (6),where F_front is a focal length of the negative lens group and F_rear isa focal length of the positive lens group.

Conditional expression (6) is a conditional expression relating to powerallocation for the negative lens group and the positive lens group. IfF_front/F_rear falls within the range in conditional expression (6), thepositive power can favorably be shared, enabling a chromatic aberration,a spherical aberration and a coma aberration to be favorably corrected.On the other hand, if F_front/F_rear exceeds the upper limit or fallsbelow the lower limit in conditional expression (6), either of the lensgroups have an excessively large power, unfavorably resulting in easyoccurrence of various aberrations.

Note that conditional expression (6), which has been described above, ispreferably:−6.0<F_front/F_rear<−1.0  (6)′,and more preferably:−4.0<F_front/F_rear<−3.0  (6)″.

The optical system for an endoscope may further meet conditionalexpressions (7) and (8):1.77<nd1<2.42  (7);and1.9<nd2<2.42  (8),where nd1 is the refractive index of the negative first lens and nd2 isa refractive index of the negative second lens.

Conditional expression (7) is a conditional expression relating to therefractive index of the negative first lens and conditional expression(8) is a conditional expression relating to the refractive index of thenegative second lens. As long as nd1 falls within the range inconditional expression (7) and nd2 falls within the range in conditionalexpression (8), the refractive indices are both are adequate, enablingprovision of a proper negative power without an extreme increase incurvatures on the object side of the negative first lens and thenegative second lens. On the other hand, if nd1 falls below the lowerlimit in conditional expression (7) or nd2 falls below the lower limitin conditional expression (8), the negative first lens or the negativesecond lens needs to have an extremely large curvature, unfavorablyresulting in easy occurrence of an off-axis aberration. Also, if nd1exceeds the upper limit in conditional expression (7) or nd2 exceeds theupper limit in conditional expression (8), unfavorably, procurement ofthe lenses themselves (or procurement of glass material) becomessignificantly difficult and high costs are needed.

Furthermore, the positive lens group in the optical system for anendoscope may include at least one positive lens, and a joined lensincluding a positive lens and a negative lens, in this order from theobject side.

The optical system for an endoscope may further meet conditionalexpression (9):0.1<PW _(—)1/PW _(—)2<0.26  (9),where PW_(—)1 is the power of the negative first lens and PW_(—)2 is thepower of the negative second lens.

Conditional expression (9) is a conditional expression relating to powerallocation in the negative lens group. As long as PW_(—)1/PW_(—)2 fallswithin the range in conditional expression (9), proper power allocationis achieved, enabling the lens diameter to be kept small while a widefield angle is maintained. On the other hand, if PW_(—)1/PW_(—)2 exceedsthe upper limit in conditional expression (9), the power of the firstlens becomes excessively large, unfavorably resulting in failure tomaintain a wide field angle as well as easy occurrence of, inparticular, an off-axis aberration. Also, if PW_(—)1/PW_(—)2 falls belowthe lower limit in conditional expression (9), the power of the secondlens becomes too large, unfavorably resulting in easy occurrence of anoff-axis aberration, and also unfavorably resulting in enlargement ofthe first lens.

Note that conditional expression (9), which has been described above, ispreferably:0.12<PW _(—)1/PW _(—)2<0.255  (9)′,and more preferably:0.15<PW _(—)1/PW _(—)2<0.25  (9)″.

All of optical elements having a power in the optical system for anendoscope may be spherical lenses.

Use of non-spherical lenses or diffractive optical elements as theoptical elements having a power results in an increase in costs.Accordingly, the optical elements having a power (optical elementsexcept, e.g., the aperture diaphragm and an optical filter) in theoptical system for an endoscope are all provided by spherical lenses,enabling the costs to be suppressed to be low.

Next, first to sixth examples of the optical system for an endoscopeaccording to the present embodiment will be described with reference toTable 1 and FIGS. 1A to 6B.

Table 1 is a table indicating values obtained according to theabove-described conditional expressions for an optical system for anendoscope according to each of the first to sixth examples. As can beseen from the table, all of the optical systems for an endoscopeaccording to the first to sixth examples meet conditional expressions(1) to (9) (further meet respective conditional expressions (1)′, (1)″,(2)′, (2)″, (3)′, (3)″, (4)′, (4)″, (6)′, (6)″, (9)′ and (9)″).

TABLE 1 ex1 ex2 ex3 ex4 ex5 ex6 H(100) × 0.150 0.195 0.308 0.247 0.3560.343 (nd01 − 1)/ Rf01 enp/FL 1.453 1.469 1.503 1.492 1.476 1.515H(100)/FL 1.951 1.994 2.097 2.046 1.874 1.911 (r1 + r2)/ 1.355 1.4231.552 1.495 1.621 1.663 (r1 − r2) |EX(ω)|/ 0.0018 0.0020 0.0022 0.00260.0004 0.0003 ω F_front/ −3.116 −3.224 −3.509 −3.298 −3.686 −3.894F_rear nd1 1.770 1.883 2.170 2.002 2.420 2.420 nd2 2.003 2.003 2.0032.003 2.003 2.420 PW_1/PW_2 0.151 0.175 0.216 0.198 0.237 0.249

Configurations of the optical systems for an endoscope according to thefirst to sixth examples along respective optical paths are illustratedin FIGS. 1A, 2A, 3A, 4A, 5A and 6A, respectively. Also, diagrams ofaberrations in the optical systems for an endoscope according to thefirst to sixth examples are illustrated in FIGS. 1B, 2B, 3B, 4B, 5B and6B, respectively.

Here, in the diagrams of the configurations of the optical systems foran endoscope along the respective optical paths, symbol L1 denotes afirst lens, symbol L2 denotes a second lens, symbol S denotes anaperture diaphragm, symbol L3 denotes a third lens, symbol L4 denotes afourth lens, symbol OF denotes an optical member contemplated to be anoptical filter such as a laser cut filter, an infrared cut filter or anoptical low-pass filter, symbol L5 denotes a fifth lens, symbol L6denotes a sixth lens, symbol L7 denotes a seventh lens, symbol CGdenotes an image pickup device cover glass, symbol AL denotes anadhesion layer, and symbol SG denotes an image pickup device chipsealing glass, respectively.

Furthermore, in optical surfaces in the first to sixth examples, r1 isan object side of the first lens L1, r2 is an image side of the firstlens L1, r3 is an object side of the second lens L2, r4 is an image sideof the second lens L2, r5 is the aperture diaphragm S, r6 is an objectside of the third lens L3, r7 is an image side of the third lens L3, r8is an object side of the fourth lens L4, r9 is an image side of thefourth lens L4, r10 is an object side of the optical member OF, r11 isan image side of the optical member OF, r12 is an object side of thefifth lens L5, r13 is a surface of joining between the fifth lens L5 andthe sixth lens L6, r14 is a surface of joining between the sixth lens L6and the seventh lens L7, r15 is an image side of the seventh lens L7,r16 is an object side of the image pickup device cover glass CG, r17 isan image side of the image pickup device cover glass CG (object side ofthe adhesion layer AL), r18 is an object side of the image pickup devicechip sealing glass SG (image side of the adhesion layer AL), and r19 isan image surface IM, which is an image side of the image pickup devicechip sealing glass SG, and the numbers provided to the respectiveoptical surfaces are surface numbers of the respective optical surfaces.Likewise, the numbers provided to the surface distances d in FIGS. 1A to6A are the surface numbers of surfaces on the object side of therespective surface distances d.

The optical systems for an endoscope according to the first to sixthexamples of the present embodiment each include a first lens group G1,which is a negative lens group, the aperture diaphragm S, and a secondlens group G2, which is a positive lens group, in this order from theobject side. Furthermore, the first lens group G1, which is a negativelens group, includes the first lens L1, which is a negative meniscuslens with a convex surface directed to the object side, and the secondlens L2, which is a negative lens, in this order from the object side.Here, the second lens L2 is a biconcave lens in the first to fourthexamples, and a negative meniscus lens with a convex surface directed tothe object side in the fifth and sixth examples.

Also, the second lens group G2, which is a positive lens group, includesthe third lens L3, which is a positive meniscus lens with a convexsurface directed to the image side, the fourth lens L4, which is aplano-convex lens (positive lens) with a convex surface directed to theimage side, the fifth lens L5, which is a biconvex lens (positive lens),the sixth lens L6, which is a biconcave lens (negative lens) and theseventh lens L7, which is a biconvex lens (positive lens), in this orderfrom the object side, and the optical member OF is disposed between thefourth lens L4 and the fifth lens L5.

Here, the fifth lens L5 and the sixth lens L6 are joined to each othervia the optical surface r13, and also, the sixth lens L6 and the seventhlens L7 are joined to each other via the optical surface r14, and as aresult, the three lenses, i.e., the fifth to seventh lenses L5 to L7form a joined lens. Consequently, the second lens group G2, which is apositive lens group, can be regarded as including at least one positivelens, and a joined lens including a positive lens and a negative lens.

All of the first to seventh lenses L1 to L7 are spherical lenses.

First Example

FIG. 1A is a diagram illustrating a configuration of the optical systemfor an endoscope according to a first example, along an optical path,and FIG. 1B includes diagrams illustrating a spherical aberration, anastigmatic aberration, a distortion aberration and a chromaticaberration of magnification in the optical system for an endoscopeaccording to the first example.

Here, in each of the spherical aberration diagram and the chromaticaberration of magnification diagram in the aberration diagrams, thealternate long and short dash line indicates a g-line (wavelength:435.84 nm), the solid line indicates a d-line (wavelength: 587.56 nm)and the dotted line indicates a C-line (wavelength: 656.27 nm),respectively. Also, in the astigmatic aberration diagram in theaberration diagrams, the dotted line indicates a meridional line M andthe solid line indicates a sagittal line S, respectively. Note that FIYin the aberration diagrams represents a half field angle and FNO. in theaberration diagrams and the below numeral data represents an F-number.

Numerical data of the optical system according to the first example areindicated below.

Surface Curvature Surface Refractive Abbe's number radius distance indexnumber νd 1 4.142 0.30 1.77000 73.00 2 0.625 0.29 3 −2.011 0.25 2.0033028.27 4 1.378 0.50 5 (Diaphragm) 0.40 6 −2.094 0.50 1.88300 40.76 7−1.186 0.05 8 ∞ 0.65 1.64769 33.79 9 −1.721 0.05 10 ∞ 0.60 1.51965 75.0111 ∞ 0.05 12 2.478 0.88 1.72916 54.68 13 −1.148 0.30 1.92286 18.90 141.812 0.60 1.88300 40.76 15 −7.685 0.25 16 ∞ 0.50 1.51633 64.14 17 ∞0.01 1.51000 63.01 18 ∞ 0.50 1.51633 64.14 19 ∞ Image surface Focallength 0.41 FNO. 5.49 Field angle 2ω 200.00 Flange focal length 0.90 (inthe air) Total length 6.32 (in the air)

Second Example

FIG. 2A is a diagram illustrating a configuration of the optical systemfor an endoscope according to a second example, along an optical path,and FIG. 2B includes diagrams illustrating a spherical aberration, anastigmatic aberration, a distortion aberration and a chromaticaberration of magnification in the optical system for an endoscopeaccording to the second example.

Here, in each of the spherical aberration diagram and the chromaticaberration of magnification diagram in the aberration diagrams, thealternate long and short dash line indicates a g-line (wavelength:435.84 nm), the solid line indicates a d-line (wavelength: 587.56 nm)and the dotted line indicates a C-line (wavelength: 656.27 nm),respectively. Also, in the astigmatic aberration diagram in theaberration diagrams, the dotted line indicates a meridional line M andthe solid line indicates a sagittal line S, respectively. Note that FIYin the aberration diagrams represents a half field angle and FNO. in theaberration diagrams and the below numeral data represents an F-number.

Numerical data of the optical system according to the second example areindicated below.

Surface Curvature Surface Refractive Abbe's number radius distance indexnumber νd 1 3.579 0.30 1.88300 40.76 2 0.625 0.29 3 −2.546 0.25 2.0033028.27 4 1.228 0.50 5 (Diaphragm) 0.40 6 −1.960 0.50 1.88300 40.76 7−1.165 0.05 8 ∞ 0.65 1.64769 33.79 9 −1.706 0.05 10 ∞ 0.60 1.51965 75.0111 ∞ 0.05 12 2.391 0.88 1.72916 54.68 13 −1.130 0.30 1.92286 18.90 141.525 0.60 1.88300 40.76 15 −8.156 0.25 16 ∞ 0.50 1.51633 64.14 17 ∞0.01 1.51000 63.01 18 ∞ 0.50 1.51633 64.14 19 ∞ Image surface Focallength 0.40 FNO. 5.49 Field angle 2ω 202.00 Flange focal length 0.89 (inthe air) Total length 6.31 (in the air)

Third Example

FIG. 3A is a diagram illustrating a configuration of the optical systemfor an endoscope according to a third example, along an optical path,and FIG. 3B includes diagrams illustrating a spherical aberration, anastigmatic aberration, a distortion aberration and a chromaticaberration of magnification in the optical system for an endoscopeaccording to the third example.

Here, in each of the spherical aberration diagram and the chromaticaberration of magnification diagram in the aberration diagrams, thealternate long and short dash line indicates a g-line (wavelength:435.84 nm), the solid line indicates a d-line (wavelength: 587.56 nm)and the dotted line indicates a C-line (wavelength: 656.27 nm),respectively. Also, in the astigmatic aberration diagram in theaberration diagrams, the dotted line indicates a meridional line M andthe solid line indicates a sagittal line S, respectively. Note that FIYin the aberration diagrams represents a half field angle and FNO. in theaberration diagrams and the below numeral data represents an F-number.

Numerical data of the optical system according to the third example areindicated below.

Surface Curvature Surface Refractive Abbe's number radius distance indexnumber νd 1 2.891 0.30 2.17000 33.00 2 0.625 0.29 3 −84.848 0.25 2.0033028.27 4 0.860 0.50 5 (Diaphragm) 0.40 6 −2.264 0.50 1.88300 40.76 7−1.347 0.05 8 ∞ 0.65 1.64769 33.79 9 −1.484 0.05 10 ∞ 0.60 1.51965 75.0111 ∞ 0.05 12 2.157 0.88 1.72916 54.68 13 −0.948 0.30 1.92286 18.90 141.702 0.60 1.88300 40.76 15 −9.072 0.25 16 ∞ 0.50 1.51633 64.14 17 ∞0.01 1.51000 63.01 18 ∞ 0.50 1.51633 64.14 19 ∞ Image surface Focallength 0.36 FNO. 5.49 Field angle 2ω 206.00 Flange focal length 0.89 (inthe air) Total length 6.31 (in the air)

Fourth Example

FIG. 4A is a diagram illustrating a configuration of the optical systemfor an endoscope according to a fourth example, along an optical path,and FIG. 4B includes diagrams illustrating a spherical aberration, anastigmatic aberration, a distortion aberration and a chromaticaberration of magnification in the optical system for an endoscopeaccording to the fourth example.

Here, in each of the spherical aberration diagram and the chromaticaberration of magnification diagram in the aberration diagrams, thealternate long and short dash line indicates a g-line (wavelength:435.84 nm), the solid line indicates a d-line (wavelength: 587.56 nm)and the dotted line indicates a C-line (wavelength: 656.27 nm),respectively. Also, in the astigmatic aberration diagram in theaberration diagrams, the dotted line indicates a meridional line M andthe solid line indicates a sagittal line S, respectively. Note that FIYin the aberration diagrams represents a half field angle and FNO. in theaberration diagrams and the below numeral data represents an F-number.

Numerical data of the optical system according to the fourth example areindicated below.

Surface Curvature Surface Refractive Abbe's number radius distance indexnumber νd 1 3.149 0.30 2.00178 19.32 2 0.625 0.29 3 −3.207 0.25 2.0033028.27 4 1.159 0.50 5 (Diaphragm) 0.40 6 −2.190 0.50 1.88300 40.76 7−1.346 0.05 8 ∞ 0.65 1.64769 33.79 9 −1.484 0.05 10 ∞ 0.60 1.51965 75.0111 ∞ 0.05 12 2.179 0.88 1.72916 54.68 13 −0.966 0.30 1.92286 18.90 141.182 0.60 1.88300 40.76 15 −7.929 0.25 16 ∞ 0.50 1.51633 64.14 17 ∞0.01 1.51000 63.01 18 ∞ 0.50 1.51633 64.14 19 ∞ Image surface Focallength 0.38 FNO. 5.47 Field angle 2ω 200.00 Flange focal length 0.89 (inthe air) Total length 6.31 (in the air)

Fifth Example

FIG. 5A is a diagram illustrating a configuration of the optical systemfor an endoscope according to a fifth example, along an optical path,and FIG. 5B includes diagrams illustrating a spherical aberration, anastigmatic aberration, a distortion aberration and a chromaticaberration of magnification in the optical system for an endoscopeaccording to the fifth example.

Here, in each of the spherical aberration diagram and the chromaticaberration of magnification diagram in the aberration diagrams, thealternate long and short dash line indicates an F-line (wavelength:486.13 nm), the dotted line indicates a d-line (wavelength: 587.56 nm)and the solid line indicates a C-line (wavelength: 656.27 nm),respectively. Also, in the astigmatic aberration diagram in theaberration diagrams, the dotted line indicates a meridional line M andthe solid line indicates a sagittal line S, respectively. Note that FIYin the aberration diagrams represents a half field angle and FNO. in theaberration diagrams and the below numeral data represents an F-number.

Numerical data of the optical system according to the fifth example areindicated below.

Surface Curvature Surface Refractive Abbe's number radius distance indexnumber νd 1 2.637 0.30 2.42000 58.00 2 0.625 0.29 3 2.034 0.25 2.0033028.27 4 0.613 0.50 5 (Diaphragm) 0.40 6 −2.877 0.50 1.88300 40.76 7−1.438 0.05 8 ∞ 0.65 1.64769 33.79 9 −1.547 0.05 10 ∞ 0.60 1.51965 75.0111 ∞ 0.05 12 1.994 0.88 1.72916 54.68 13 −0.973 0.30 1.92286 18.90 142.346 0.60 1.88300 40.76 15 −14.176 0.25 16 ∞ 0.50 1.51633 64.14 17 ∞0.01 1.51000 63.01 18 ∞ 0.50 1.51633 64.14 19 ∞ Image surface Focallength 0.35 FNO. 5.50 Field angle 2ω 202.00 Flange focal length 0.89 (inthe air) Total length 6.31 (in the air)

Sixth Example

FIG. 6A is a diagram illustrating a configuration of the optical systemfor an endoscope according to a sixth example, along an optical path,and FIG. 6B includes diagrams illustrating a spherical aberration, anastigmatic aberration, a distortion aberration and a chromaticaberration of magnification in the optical system for an endoscopeaccording to the sixth example.

Here, in each of the spherical aberration diagram and the chromaticaberration of magnification diagram in the aberration diagrams, thealternate long and short dash line indicates an F-line (wavelength:486.13 nm), the dotted line indicates a d-line (wavelength: 587.56 nm)and the solid line indicates a C-line (wavelength: 656.27 nm),respectively. Also, in the astigmatic aberration diagram in theaberration diagrams, the dotted line indicates a meridional line M andthe solid line indicates a sagittal line S, respectively. Note that FIYin the aberration diagrams represents a half field angle and FNO. in theaberration diagrams and the below numeral data represents an F-number.

Numerical data of the optical system according to the sixth example areindicated below.

Surface Curvature Surface Refractive Abbe's number radius distance indexnumber νd 1 2.811 0.30 2.42000 58.00 2 0.700 0.29 3 1.415 0.25 2.4200058.00 4 0.544 0.50 5 (Diaphragm) 0.40 6 −3.332 0.50 1.88300 40.76 7−1.329 0.05 8 ∞ 0.65 1.64769 33.79 9 −1.698 0.05 10 ∞ 0.60 1.51965 75.0111 ∞ 0.05 12 2.059 0.88 1.72916 54.68 13 −0.948 0.30 1.92286 18.90 143.307 0.60 1.88300 40.76 15 −10.876 0.25 16 ∞ 0.50 1.51633 64.14 17 ∞0.01 1.51000 63.01 18 ∞ 0.50 1.51633 64.14 19 ∞ Image surface Focallength 0.36 FNO. 5.50 Field angle 2ω 206.00 Flange focal length 0.89 (inthe air) Total length 6.31 (in the air)

According to such embodiment, in an optical system for an endoscopehaving a field angle of no less than 200 degrees, a distance from anegative first lens to an aperture diaphragm is reduced to move aposition of an entrance pupil to the object side, enabling reduction indiameter of the negative first lens. Furthermore, an amount ofrefraction of a surface on the object side of the negative first lens isproperly determined for an incident angle that corresponds to a halffield angle of 100 degrees, enabling occurrence of various aberrationsto be favorably suppressed.

Consequently, a small-diameter optical system for an endoscope that issuitable for use in an endoscope (for example, a urological endoscope orany of other endoscopes) can be provided.

Note that the present invention is not limited to the above-describedembodiment as it is, and in the practical phase, the present inventioncan be embodied with components modified without departing from thespirit of the present invention. Furthermore, various aspects of theinvention can be provided by arbitrary combinations of a plurality ofcomponents disclosed in the above-described embodiment. For example,several components may be deleted from all the components indicated inthe embodiment. Furthermore, components may arbitrarily be combinedthrough the different embodiments. As described above, it should beunderstood that various modifications and applications are possiblewithout departing from the spirit of the invention.

What is claimed is:
 1. An optical system for an endoscope, the opticalsystem comprising: a negative lens group, an aperture diaphragm and apositive lens group in this order from an object side, wherein thenegative lens group includes a negative first lens and a negative secondlens in this order from the object side; and wherein the optical systemmeets conditional expressions (1) and (2):0.05<H(100)×(nd01−1)/Rf01<0.38  (1)and0.5<enp/FL<2.5  (2), where: H(100) is a height at which a principal raywith a field angle of incidence of 100 degrees falls on a surface on theobject side of the first lens; Rf01 is a curvature radius of the surfaceon the object side of the first lens; nd01 is a refractive index of thefirst lens; FL is a focal length of an entire system; and enp is adistance between an entrance pupil and the surface on the object side ofthe first lens.
 2. The optical system for an endoscope according toclaim 1, wherein the optical system further meets conditionalexpressions (3) and (4):0.5<H(100)/FL<3.5  (3);and1.1<(r1+r2)/(r1−r2)<1.9  (4), where: r1 is the curvature radius of thesurface on the object side of the first lens; and r2 is a curvatureradius of a surface on an image side of the first lens.
 3. The opticalsystem for an endoscope according to claim 1, wherein where a lenssystem incident angle ω=100, the optical system further meetsconditional expression (5):0.00<|EX(ω)|/ω<0.03  (5), where EX(ω) is an angle at which a principalray incident at the incident angle co exits from the optical system. 4.The optical system for an endoscope according to claim 1, wherein theoptical system further meets conditional expression (6):−8.0<F_front/F_rear<−0.1  (6), where F_front is a focal length of thenegative lens group and F_rear is a focal length of the positive lensgroup.
 5. The optical system for an endoscope according to claim 1,wherein the optical system further meets conditional expressions (7) and(8):1.77<nd1<2.42  (7);and1.9<nd2<2.42  (8), where nd1 is the refractive index of the negativefirst lens and nd2 is a refractive index of the negative second lens. 6.The optical system for an endoscope according to claim 1, wherein thepositive lens group includes at least one positive lens and a joinedlens including a positive lens and a negative lens, in this order fromthe object side.
 7. The optical system for an endoscope according toclaim 1, wherein the optical system further meets conditional expression(9):0.1<PW _(—)1/PW _(—)2<0.26  (9), where PW_(—)1 is a power of thenegative first lens and PW_(—)2 is a power of the negative second lens.8. The optical system for an endoscope according to claim 1, wherein allof optical elements having a power in the optical system for anendoscope are spherical lenses.